Mechanical pulp, most often CTMP (Chemi Thermo Mechanical Pulp), is a major component in multiply board constructions, and often constitutes about 70% of the centre layer and up to 50% of the board furnish.
To achieve high bending stiffness of the board it is desirable to have a high strength at a sheet density of the centre ply as low as possible. A favourable Scott bond-density relationship is thus essential. To this end, there are two opposing aspects that have to be considered. Firstly, only the components of the raw material which contribute to strength and bulk are desirable to have in the refined pulp. The other components, which do not contribute to strength and bulk in any extent, basically constitutes a load in the system, and require additional drying capacity, bleaching agent consumption, etc. On the other hand, every component that remain in the pulp will contribute to a high yield in the production. One of the aspects that renders mechanical pulp competitive in relation to chemical pulp, is the higher yield of mechanical pulp.
Another important aspect of the mechanical pulp is the taint and odour properties, which are directly related to the extractives and manganese left in the pulp. The taint and odour properties are especially important when the pulp is to be used in paperboard for food or liquid related applications. This is true for all types of mechanical pulp. For TMP (Thermo Mechanical Pulp), the taint and odour problem is so serious that this type of pulp is not used in paperboard today. If the pulp is to be bleached with non-chlorine bleaching agents, especially peroxide containing bleaching agents, the remaining manganese content in the pulp constitutes a problem, which results in a higher consumption of bleaching agents and an increased need for complexing agents, such as EDTA or DTPA, in order to reach a desired brightness level. Also, a remaining extractives content in the pulp results in a high washing demand, and increased contamination of the water system and the effluents from the mill.
It has been shown in laboratory studies for bleached kraft pulps that primary fines have detrimental effects on the bleachability of the pulp, “Bäckström, M., Htun M.: Improved Bleachability Without Primary Fines, 1998 International Pulp Bleaching Conference, Proceedings Book 2, pp 333–336”. Similar results on the bleachability of kraft pulp have been reported in a study on the effects of ray cells, “Westermark, U., Capretti, G.: Influence of Ray Cells on The Bleachability and Properties of CTMP and Kraft Pulps, Nordic Pulp & Paper Res. J. 3(1988) 2, pp 95–99”. In the latter reference, the effect of ray cells in mechanical pulps has also been studied. It was concluded that no significant difference in bleachability for CTMP pulp with and without ray cells, respectively, could be detected. As to the contribution of ray cells on strength and bonding properties of CTMP and TNT pulp, it was concluded that an addition to the pulp of a ray cell fraction, resulted in poor bonding and strength properties.
In “Rydholm, S. A.: Pulping Processes, Interscience, New York (1965), p 744”, it has been suggested to use specially designed so called ray cell filters for the separation of resin containing ray cells in the sulphite pulp industry.
For mechanical pulps however, no industrial process has been designated to address the problems related to primary fines, which problems are specific for mechanical pulps, i.e. problems with taint, odour, strength and bulk. Neither has a coarse pulp fraction, free from primary fines, been treated to attain improved pulp properties for the production of paper or paperboard.